The present invention relates to a fuel control system for an internal combustion engine. The invention is particularly concerned with a closed loop lean mixture control system responsive to a signal from an oxygen concentration sensor provided in the exhaust system of the engine.
Use has extensively been made of oxygen sensors of the type wherein a current is generated as an indication of oxygen content when a predetermined voltage is applied. With the application of a 0.6-volt potential, the sensor generates a current which varies linearly from zero to 10 milliamperes or higher as a function of oxygen content which varies from zero to 10 percent upward. The current represents a range of air-fuel ratios which correspond to lean mixtures. This occurs only when the sensor is operating at a temperature higher than a predetermined threshold. If the temperature in the sensor's environment is lower than the threshold, the current is not a true indication of oxygen content and this occurs when the engine is started.